This is a very difficult question to answer with any authority, because readers will perhaps differ about when the line between survival and savagery is crossed in this novel. William Golding's Lord of the Flies explores what happens when humans (in this case, English schoolboys) are forced to live without any form of law or authority other than that which they create themselves.
The first death on the island is an accident; the boy with the mulberry birthmark is consumed by the out-of-control fire the boys set. The second death may mark the line between survival and savagery, as both elements are part of the action.
In chapter nine, Jack and his hunters have just killed a pig and are celebrating with a great feast. A terrible storm is brewing, and Simon has made a discovery he needs to share with the others. As all the boys but Simon are gathered in a circle chanting "Kill the beast! Cut his throat! Spill his blood! Do him in!," Simon crawls weakly from the forest after his encounter with the pig's head (referred to as the Lord of the Flies), and the others are caught up in the frenzy of reenacting their successful hunt and circle Simon as if he were a pig before they kill him.
The next morning, Ralph and Piggy are ashamed and cannot talk about the incident; even Jack is a bit subdued, as if he might still have some sense of conscience about the heinous act.
Simon's killing is not for survival but is caused by the naked savagery of the boys. They certainly had no intention of eating him for survival, and Simon clearly had no intention of harming them. If this is not the moment where the line is crossed, Piggy's death certainly would be it. Boys deliberately drop a boulder on him because they do not like him--and because they can.